Light emitting diodes (LEDs) are employed in a wide variety of applications. For example, in optical data transmission, LEDs are used to launch data signals along a fiberoptic cable.
Unlike lasers, LEDs do not generate well-focused beams of light. Rather, an LED radiates light in all directions. That is, the light emission is isotropic. The layers of many conventional LEDs are grown on an optically absorbing substrate having an energy gap less than the emission energy of the active region of the LED. The substrate absorbs some of the light generated within the active region, thereby reducing the efficiency of the device. An example of a prior art aluminum gallium arsenide (AlGaAs) LED of the single heterojunction type is shown in FIG. 1. An epitaxial layer 10 of p-doped AlGaAs and an epitaxial layer 12 of n-doped AlGaAs are grown on a surface of a p-doped gallium arsenide (GaAs) substrate 14. The conduction of current through the junction of the epitaxial layers 10 and 12 will generate light. However, since the energy gap of the absorbing substrate 14 is less than the emission energy, light that is emitted or internally reflected downwardly toward the substrate 14 will be absorbed.
FIG. 2 is a double heterojunction AlGaAs LED on an absorbing substrate 16. An epitaxial layer 18 of n-doped AlGaAs and two layers 20 and 22 of p-doped AlGaAs are grown on the absorbing substrate 16. The bandgaps of the epitaxial layers 18-22 are chosen to cause light to be generated in the active layer 20 and to travel through the epitaxial layers 18 and 22 without being absorbed. However, absorption of light does occur at the substrate 16.
Improved performance can be achieved by employing a transparent substrate that has an energy gap greater than the emission energy of the LED active region. The effect of the transparent substrate is to prevent the downwardly emitted or directed light from being absorbed. Rather, the light passes through the transparent substrate and is reflected from a bottom metal adhesive and reflecting cup. The reflected light is then emitted from the top or the edges of the chip to substantially improve the efficiency of the LED.
There are several techniques for fabricating LEDs having transparent substrates. A first technique is to epitaxially grow the p-n junction on a transparent substrate. However, a problem with this technique is that acceptable lattice matching may be difficult to achieve, depending upon the lattice constant of the LED epitaxial layers. A second technique is to grow the LED epitaxial layers on an absorbing substrate that is later removed. For example, in FIG. 3 the n-doped transparent substrate 24 and the p-doped epitaxial layers 26 and 28 may be epitaxially grown on an absorbing substrate, not shown. The transparent "substrate" 24 is fabricated by growing a thick, greater than 75 .mu.m, optically transparent and electrically conductive epitaxial layer on the lattice-matched absorbing substrate. The other layers 26 and 28 are then grown on the epitaxial "substrate" 24 and the absorbing substrate is removed. Alternatively, the thinner layers 26 and 28 may be grown before the thicker transparent "substrate" 24.
The above-described techniques of fabricating LEDs having transparent substrates suffer from inherent disadvantages. Firstly, epitaxially growing a "thick" optically transparent, electrically conductive "substrate" may not be practical, or even possible, when employing some growth techniques for certain semiconductor materials. Secondly, even when possible, a "thick" epitaxial layer requires a long growth time, limiting the manufacturing throughput of such LEDs. Thirdly, following removal of the absorbing substrate, the resulting LED layer is relatively thin, e.g. approximately 3-6 mils. The thin wafers are difficult to handle without breaking, rendering fabrication more difficult. Moreover, thin wafers create difficulties during mounting the wafers in an LED package. Silver-loaded epoxy is typically utilized for mounting and contacting the bottom of the device. The epoxy tends to flow over the edges of thin wafers, causing the short circuiting of the diode (LED). Also, thin wafers are not as mechanically robust as the devices of FIGS. 1 and 2, which are grown on "thick" substrates of at least 10 mils. Such "thin" LEDs may exhibit increased device-failure problems when mounted in epoxy lamps. Thus, there are contradictory thickness problems when this second technique is employed, since the transparent layer may be "too thick" for practical crystal growth processes and "too thin" for device applications.
Consequently, there may be a tradeoff associated with selection of an absorbing substrate or a transparent substrate. Depending upon the growth and fabrication techniques, an LED having an absorbing substrate may possess mechanical characteristics that are superior to a transparent substrate LED, but the absorbing substrate LED is generally less efficient. Increased efficiency is possible using a transparent substrate; however, lattice mismatch may create difficulties when the epitaxial layers are grown on a transparent substrate having a different lattice constant. In addition, the contradictory thickness problems may be encountered when a "thick" transparent "substrate" is epitaxially grown.
The effect of an absorbing layer or substrate can be minimized by growing a Bragg reflector between the standard LED epitaxial layers and the absorbing substrate. An increase in efficiency is achieved, since the Bragg reflector will reflect light that is emitted or internally reflected in the direction of the absorbing substrate. However, the improvement is limited compared to transparent substrate techniques, because the Bragg reflector only reflects light that is of near normal incidence. Light that differs from a normal incidence by a significant amount is not reflected and passes to the substrate, where it is absorbed. Moreover, LEDs having Bragg reflectors are more difficult to manufacture, since they require the repeated growth of many thin epitaxial layers, typically on the order of 100 angstroms in thickness.
It is an object of the present invention to provide a method of forming an LED having the desirable mechanical characteristics of a "thick" substrate of at least 8 mils and the desirable optical characteristics of a transparent-substrate LED.